How To Build A Stone Foundation | COB HOUSE Part 2

In this tutorial series, we're learning how to build a cob house from the foundation all the way through the reciprocal living roof. And in the first episode, we went through a walkthrough of the building, covered some basic design principles, and explored the stages of the building process. In this episode, we'll be learning about how to build a rubble trench foundation, which is a really simple and effective method to create a strong and long-lasting foundation, which combines drainage and load-bearing functions. As we mentioned in the first episode, this Cruising Cob Global Workshop took place over a span of five weeks in the North Carolina mountains. On the first day of the workshop, we all introduced ourselves for the first time, we prepared one of many organic meals that we'd be sharing together over the coming five weeks, and then we took a guided tour of the property together. After the tour, we had a brief opening circle where we all shared our intentions for coming to the workshop and we got to know each other a little bit. After dinner, we all set up camp and got ready for the big day tomorrow, where we'd be breaking ground for the first time. When choosing your building site, you should pick an area that is well draining with a solid and homogeneous subsurface geology. Luckily for us, the soil we excavated from this site was a stable and sturdy rich red clay that we actually mixed right back into the cob that created the walls of this home. In the weeks leading up to the workshop, the instructors actually completed the first step of building a rubble trench foundation, which is to remove all the vegetation, topsoil, and organic matter from the building site. They rented excavating equipment to grade the building site down to a slight slope, which is ideal for building a cob cottage because your rubble trench foundation needs to drain slightly downwards until it reaches daylight, which would be really difficult to achieve on perfectly flat land. They also completed step two, which is marking the perimeter of your building, and in this case they used a center peg with seven and a half and eight and a half feet pieces of string to mark an outline of our rubble trench, which would end up giving us a 15-foot diameter interior space for this cottage. We enjoyed our breakfast, the goats enjoyed their breakfast, and after a quick stretch, we were finally ready to break ground on this building. We used many different tools for digging, a trenching spade, lots of shovels, hoes, mattocks, and a post hole digger, and we chose to dig a one-foot wide rubble trench because we'd be building a one-foot wide stone stem wall on top of the trench that would then support a one-foot wide cob wall at the base. The trench should slope a minimum of one inch for every four feet of distance, and it's also important to take into consideration the frost line in your area to determine the proper depth of the trench. In our case, the frost line is only six inches, but we chose to dig a depth of one foot because we would be stacking this stem wall starting about six inches below ground level, and this would ensure that we would still have enough room for proper drainage beneath the stem wall. As we continued digging, we used these 12-inch pieces of wood to ensure that we were getting a proper depth and proper width of our trench. We also continued using the string line to our center peg with a can of spray paint to make sure that we were staying in line with our measurements. We dug the shape of the trench to resemble an elephant foot that was actually two inches wider at the bottom to improve stability, and we also had a low point to allow for proper drainage. Here we're digging the exit trench on the low side of the building where any water that enters the foundation will be able to exit safely. We used levels to make sure we had achieved our desired slope all the way around the circle, and then after a few more final touches, we were ready to put our trench to the test. All right guys, let's do it. Yeah, let's do it. One, two, three. We took a break for lunch and had lecture, and then we were ready to start gathering the drainage rock to fill the rubble trench. You want to use a clean drainage rock in between three-quarter inches and four inches in diameter. We're using three-quarter inch clean stone, which can be bought by the cubic yard from most landscape supply companies. You also want to line the trench with landscaping fabric before filling with drain rock. This will allow the water into the trench, but will prevent the surrounding soil from seeping into the trench, which could eventually cause drainage issues. As you fill the trench with drainage rock, you want to pour in maximum increments of six inches, spreading evenly and tamping in between each layer. You don't want your foundation to eventually settle, which could lead to cracking in your cob wall. Here we're laying some corrugated pipe into the exit trench, and I've seen many builders that lay this pipe throughout their entire foundation, but for this build we only used it here. We also tied some more landscape fabric on the end to make sure this pipe wouldn't fill with gravel, and then Percy came by to check everything out and make sure we were doing a good job. We used compacting the gravel as an excuse to do a few dance moves and a couple waves, and then we compacted the gravel on top of the drainage pipe in the exit trench as well. Once the drainage rock was all compacted, we could then begin folding both sides of the extra landscaping fabric on top, and then we buried the exit trench with the surrounding soil. For the rest of the trench, we tucked the inside layer first, and then the outside layer on top. The final step of day two was putting just enough gravel on top of the landscape fabric to hold everything in place. Coming into day three, we prepared to start building our mainly dry stacked stone stem wall. The stem wall is the portion of the foundation that rises above ground level, and its purpose is to raise the cob walls high enough so that they won't become saturated from water runoff or splashback when it rains. We sourced many of the foundation stones directly from the property, but we also brought in some stone from a nearby landscaping supply company as well. As we previously mentioned, we began stacking the stone stem wall about six inches below ground level. This allows the stones to key into the ground and keeps them held firmly in place. You want to pick out stones that have as many flat faces as possible to make them easier to stack, and you also want to use stones that are structurally strong and solid, since they will be supporting the entire weight of the building. Typically it's common practice in the natural building world to use stone for the stem wall, but there are other materials such as concrete block, a continuous concrete pour, earth bags, or urbanite that can also be used. In general, natural builders try to avoid using excessive amounts of concrete and cement in their buildings due to the extremely high embodied carbon footprint created in the manufacturing process. You want to lay out your stones nearby and familiarize yourself with their shapes so that you can choose ones as you need them without searching for very long. If you're struggling to find a certain shape, it's also possible to modify the shape of the stone with a hammer and chisel. You also want to put most of your largest stones in the bottom layer and continue checking your measurements with your string line to ensure that you're staying on course. As you continue stacking, you can fill in small gaps with gravel to increase the stability between the joints. This is about to where we got before taking another break for lunch. In general, when you're stacking stones, the stone that you place on top should distribute its weight to at least two stones underneath of it and you want to apply pressure to make sure that it's a sturdy and solid fit. Another great way to further test this is to actually walk on top of the stones as you place them in each run. We mainly tried to stack the stones in roughly equal runs so that we could add cement mortar to areas that seemed like they needed a little bit more rigidity. Even if you are using mortar, the stones should be stacked together solidly so that if there were no mortar they would still be stable and structurally strong. This is where we were at at the end of day three and we finished the day by sharing some time around the fire together. Coming into day four, we took some time to review our work from yesterday and we took notes of areas where our work was strong and also some areas that would need to be redone. It's really important to get the foundation right since your entire building will be resting on top of it and you can't easily go back and make changes once you start building your walls. We continued bringing in large stones and dispersing them evenly around the foundation. It's okay if your stem wall becomes slightly wider than your intended cob wall width, but you do want to be intentional about the slopes on the stones that you leave exposed. For stones that reach slightly beyond the footprint of the trench, you want to be sure that the exterior edge slopes downward and away from the building. This will allow any sideways rain that does happen to hit the walls of the building to have a way to drip off of the foundation. It's a good idea to continue using your string line to make sure that everything is within your desired range. Similarly to how bricks are laid, you want to avoid vertical seams that run all the way through each course and instead you want to stagger the joints whenever possible. We originally planned to mortar some smaller flat stones together to create a threshold for the doorway, but then we came across this beautiful stone bench on the property and we thought it might be a perfect fit. It needed to be about six inches tall to meet the top of the earthen floor that we'd be pouring further into the build and after measuring, it was indeed perfect. Four of us wheeled it back to the building site on the dolly and like magic, it fell right into place. You can see that some of the larger stones slant from side to side at the top of the foundation and this is actually ideal because when you start building your cob walls, this will give the cob something to key into. You would really want to avoid sharp slants that go towards the interior or the exterior of the building though because this would encourage the cob to slide off of the foundation and could even be structurally problematic. Another consideration is to place flat and aesthetically pleasing stone faces towards the exterior and the interior that will still be exposed once the building is complete. An exciting part of day four was installing the door frame on top of our new stone threshold. Josh built this door frame with some two by sixes and he purposefully built an extended header into the frame that would help distribute the weight from the cob and roof on top down into the walls beside the door. The size of your door and window headers is really dependent on how wide they span, how much cob they will be supporting above, the weight of your roof, and so on. So if you're planning on taking on a project of your own, it could be a good idea to consult with an experienced natural builder on your design or even get your plans approved by a structural engineer. Since we would have less than six inches of cob above our door and window frames, these four inch overhangs in our headers would suffice and we wouldn't have to go back and add additional headers on top. We also distributed the weight of the primary rafters in the roof so that they wouldn't rest directly over top of door and window frames, but we'll get more into that in a future episode. You can see that he built the frame with supports in each corner to keep it square and after checking with the level, we used stakes and supports to hold it level and plumb that would eventually be removed once the cob walls took hold of the door frame. As day four came to a close, we celebrated another day of good work and later that night we got to enjoy a show put on by the fireflies. The morning of day five would be our last and final time spent on building the rubble trench foundation. We continued going around and testing each area of the stem wall to find areas that needed adjustments. For the entire stem wall, we built to a minimum height of 12 inches from ground level. On the high side of the foundation was where we met this 12 inch minimum and then we built the low side of the foundation up to be roughly level all the way around the foundation. This actually caused sections of the lower end of the foundation to be up to a foot and a half or even two feet tall in some areas. As we continued working our way around and leveling everything out, we added a little bit more mortar to areas that we thought might need it. We also continued using our string line as one last final check to make sure that everything was in line. The mortar that we used was just a basic mixture of cement, sand, and water. At the very top of the stem wall, we used the same mortar and some pointier stones to create some more teeth for the cob to grip into. Something I haven't yet mentioned was the purpose of the temporary tarp structure, and on day five we saw our first day of rain. This tarp allowed us to keep the building site and ourselves dry as we continued working. The tarp actually becomes more important during the cobbing process because you want to be able to blast the cob walls with full sun on dry days and to keep them covered on rainy days, especially when you're on a tight schedule like this workshop was. One final consideration is that generally you want to have your grade surrounding your foundation slope away from your home. So in this case, since there's a large hill that drains towards the house to the left of the entrance, the owners should go back and install a secondary French drain on that side of the building to divert water away from the foundation. The last thing we did at the end of the foundation was to move all the stones to one spot and to clean up the building site as we prepared to start building our earthen floor, which will be covered in the next episode. If you're enjoying the series so far, please leave a like and hit the subscribe button so that you can follow along with the rest of this build. We're also looking to connect with people who are interested in hosting natural building workshops and other natural builders who want to collaborate, so follow us on Instagram at naturalbuildings and send us a direct message if you'd like to learn more.